The invention relates to a method and a device for determining per breath the partial pressure of a gas component in the respiratory air of a patient, to be more exact a method for determining the CO2 content in the respiratory air, and a ventilation device which is designed such that the method can be carried out using it.
In medicine, the carbon dioxide content in respiratory air can be used to draw important conclusions concerning the state of health of a patient and possible medical conditions. In addition to the absolute content (or the partial pressure) of the carbon dioxide in the exhaled air, the CO2 curve shape is important (corresponding curves in which the CO2 content (or partial pressure) is plotted against time are referred to as capnograms), particularly also whether the maximum value of the carbon dioxide content in the exhaled air rises, falls or remains constant during the respiratory cycles and at what speed it rises or falls. These types of information are particularly important if a patient is intubated and artificially ventilated, for example under general anaesthesia, but also in emergency medicine and during spontaneous ventilation.
FIG. 1 illustrates a number of examples of typical capnograms. Of these, FIG. 1a) shows the capnogram of a healthy patient under controlled ventilation. The maximum CO2 content in the exhaled air is about 5%. FIG. 1b) shows the capnogram of a patient in whom the normal CO2 curve falls to 0 from one breath to the next. This can be caused, for example, by a disconnection of the ventilator from the patient, or there may have been a complete airway obstruction, caused for example by a completely blocked endotracheal tube. FIG. 1c) shows a rapid and constant fall of the CO2 content in the exhaled air and may be an indication of a significant pulmonary air embolism, a cardiac arrest or severe hypotension. In FIG. 1d) the carbon dioxide content in the exhaled air suddenly falls to a lower level, but not to 0, and remains constant at said level. This is caused for example by shifting of the endotracheal tube into a bronchus, for example when changing the position of the patient, or by a sudden partial airway obstruction. A capnogram also provides indications of the onset of hyperventilation, a fall in cardiac output or pulmonary perfusion, onset of hypoventilation, increasing energy conversion rate as a consequence of pain or fever, inadvertent intubation of the stomach instead of the lung, malignant hyperthermia, inadequate muscle relaxation and inadequate depth of anaesthesia and other serious or life-threatening conditions of the patient.
Even when a complete capnogram is unavailable, it is still possible to use the development of the maximum carbon dioxide content in the exhaled air over a number of respiratory cycles to obtain valuable information on possibly serious or life-threatening conditions of a patient. Corresponding trend curves, as they are known, are shown in FIG. 2. In FIG. 2a) the patient is initially stable and the maximum content of the carbon dioxide in the exhaled air is about 5%. However, the maximum content of the carbon dioxide in the exhaled air suddenly falls off rapidly. Possible causes for this are a cardiopulmonary bypass, cardiac arrest, pulmonary embolism, great loss of blood, or an extremely abrupt drop in blood pressure. FIG. 2b) shows a constantly low maximum carbon dioxide content in the exhaled air, at just under 4%. Possible causes for this are hyperventilation caused by too high a minute volume or a low body temperature following shock. FIG. 2c) shows the sudden fall in the maximum value of the carbon dioxide content in the exhaled air to about 0. Possible causes for this are accidental extubation, total airway obstruction, disconnection or oesophageal intubation. In the event of oesophageal intubation, the drop to 0 occurs after just one to two respiratory cycles. FIG. 2d) shows a gradual rise in the maximum value of the CO2 concentration in the exhaled air, possibly caused by an increase in metabolism and body temperature, incipient hypoventilation, or by a decrease in effective alveolar ventilation . FIG. 2e) shows the trend curve in the case of a sudden drop in the maximum value of the carbon dioxide content in the exhaled air, for example as a result of leakage in the tube system, a partial airway obstruction, or a tube in the hypopharynx. FIG. 2f) shows a constantly high maximum value of the carbon dioxide in the exhaled air, possible causes of which are respiratory depression caused by medication, metabolic alkalosis (respiratory compensation) or an inadequate minute ventilation.
Concerning determination of CO2 in exhaled air in medicine, reference can be made to the xe2x80x9cAnnals of Emergency Medicinexe2x80x9d 1989, 1287/53 to 1290/56, xe2x80x9cAnnals of Emergency Medicinexe2x80x9d 1989, 166/1375, xe2x80x9cPrehospital and Disaster Medicinexe2x80x9d Vol. 4, # 1, 1989, page 74, and xe2x80x9cJAMAxe2x80x9d 1987, Vol. 257, No. 4, 512 to 515.
Devices for determining the carbon dioxide content in exhaled air are known and are widely used in medicine. These devices must be able to respond rapidly to changes in the CO2 content in the exhaled air, and the devices generally used for this purpose are ones based on infrared absorption spectroscopy. Devices using carbon dioxide sensors of this type are described for example in EP-A 392 503, DE-A 35 33 557 and DE-A 31 37 258.
Devices are also known in which the CO2 content is indicated as a colour change on an indicator system. Such devices are described for example in U.S. Pat. No. 4,728,499 and are available commercially.
The CO2 detectors based on IR absorption spectroscopy have the advantage that they have a very rapid response time and reproduce the CO2 content in the exhaled air with very high resolution. Such devices are extremely expensive, however, and their use, particularly in emergency medicine, for example in ambulances, is generally not possible for reasons of cost. CO2 detectors based on a colour indicator reaction are admittedly less expensive, but they do not provide any trend information and, since colour comparisons are required, they are relatively imprecise and difficult to read off. Nor is it possible to record capnograms using CO2 detectors based on a colour indicator reaction.
There is a real need in medicine for an inexpensive device for determining the carbon dioxide content in respiratory air. The method is meant to function per breath, that is to say that the carbon dioxide content (i.e. the carbon dioxide partial pressure) is meant to be determined in the inhaled air and in the exhaled air upon each respiratory cycle. The method is meant to be able to indicate a trend curve, or at least trend information, that is to say to indicate whether the maximum carbon dioxide content in the exhaled air decreases, increases or remains the same in successive respiratory cycles, and at what speed this occurs. There is also a need for a device for carrying out such a method.
A method for determining carbon dioxide production in respiratory gas is known from DE-A 40 01 803. The intention is to dispense with a device for measuring carbon dioxide concentration. In this method, in a serial measurement cycle, two oxygen sensors are used first to measure the oxygen consumption and the oxygen concentration values, with and without CO2 absorber, and from these values, and from the oxygen concentration in the inhalation branch, the carbon dioxide production is then calculated in the control unit. The carbon dioxide content or carbon dioxide partial pressure in the exhaled air cannot be measured per breath using the device described in DE-A 40 01 803, and a corresponding method is not the subject of said publication. The determination of carbon dioxide production in respiratory gas as described in DE-A 40 01 803 must not be confused with the subject of the present invention which is not concerned with carbon dioxide production, but with the carbon dioxide content in the exhaled air, which must be determined per breath.
It is an object of the invention to make available a method and a device for determining the carbon dioxide content in respiratory air, which does not have the problems of the prior art and which in particular is so inexpensive that it can also be readily employed in ambulances and emergency medical vehicles and yet supplies reliable information on the absolute value and, above all, also on the trend of the carbon dioxide content in the respiratory air over several respiratory cycles.
This object is achieved by the subject matter of the patent claims.
The method according to the invention is based on using a rapid oxygen sensor to determine the oxygen partial pressure during breathing. The oxygen partial pressure can then be used to draw conclusions concerning the carbon dioxide content in the respiratory air. If, for example, the oxygen partial pressure in the inhaled air is 21 kPa and the oxygen partial pressure in the exhaled air falls to 16 kPa, the difference in the oxygen partial pressure of 5 kPa corresponds on first approximation to the maximum value of the carbon dioxide partial pressure in the exhaled air. By continuously measuring the oxygen partial pressure in the respiratory air, curves can be obtained which are the inverse of those shown for example in FIG. 1, and it is possible, by simple conversion, to obtain and record capnograms.
In a preferred simpler and less expensive embodiment, which is conceived in particular for use in ambulances and emergency medical vehicles, the whole capnogram is not recorded and displayed, instead only the maximum value of the carbon dioxide content in the exhaled air is determined and displayed per breath. In addition, the maximum value of the carbon dioxide content in the exhaled air is compared on each respiratory cycle with the previous respiratory cycles, and trend information is displayed, that is to say information on whether the maximum value of the carbon dioxide content in the exhaled air rises, falls or remains constant from respiratory cycle to respiratory cycle. Depending on this information, the operating personnel can then initiate appropriate measures or ask for detailed tests to be carried out.